1. Field of the Invention
The present invention relates to a light-emitting device and in particular to a light-emitting device comprising buffer multilayers of gradual lattice constants to reduce lattice mismatch in the joint of layers.
2. Description of the Related Art
Generally, light-emitting devices comprise light emitting and laser diodes. A group-III nitride semiconductor light-emitting diode is fabricated by providing an electrode on a stacked layer structure having a pn-junction type light-emitting part comprising, for example, aluminum gallium indium nitride (AlxGayIn1xe2x88x92xxe2x88x92yN, where 0 less than x, y less than 1 and 0 less than x+y less than 1). The lattice constant of the group-III nitride semiconductor layer is different from that of the substrate.
In the stacked layer structure, a buffer layer is generally provided to reduce lattice mismatch between the substrate material and the group-III nitride semiconductor layer constituting the stacked layer structure, thereby forming a high-quality group-III nitride semiconductor layer. Conventionally, for example, in the stacked layer structure for use in a light-emitting device using a sapphire (xcex1-Al2O3 single crystal) substrate, the buffer layer is exclusively composed of aluminum gallium nitride (compositional formula; Alxcex1Gaxcex2N, where 0xe2x89xa6xcex1, xcex2xe2x89xa61).
In FIG. 1, a conventional light-emitting device having a buffer layer is shown. The light-emitting device 10 comprises sapphire as a substrate 100. The stacked layers are formed on the c {0001} plane of the sapphire substrate 100. The stacked layers comprise a buffer layer 102, a first group-III nitride semiconductor layer 104, a first cladding layer 106 consisting of a hexagonal GaN, an active layer 108 consisting of a GaN-based semiconductor layer, a second cladding layer 110, and a second group-III nitride semiconductor layer 112. The buffer layer 102 relaxing lattice mismatch between the substrate and the following stacked layer usually consists of GaN. A first type electrode 116 is deposited on the first group-III nitride semiconductor layer 104, doped with the same conductive type as the first type electrode 116. A second type electrode 114 is deposited on the second group-III nitride semiconductor layer 112, doped with the same conductive type as the first type electrode 114.
However, lattice mismatch between the sapphire substrate and the group-III nitride semiconductor layer is still high (about 13.8%), causing defects in the stacked layer. Lifetime is shortened and light-emitting efficiency is decreased thereby, making it necessary to solve the lattice mismatch.
Accordingly, an object of the invention is to provide a light-emitting device with reduced lattice mismatch, thereby improving light-emitting efficiency.
One feature of the present invention is use of a buffer multilayer comprising a first buffer multilayer, such as BxGa(1xe2x88x92x)P, and a second buffer multilayer, such as InyGa(1xe2x88x92y)N. The buffer multilayer is deposited between the substrate and the first cladding layer. The lattice constant of the bottom layer of the first buffer multilayer is substantially equal to the lattice constant of the substrate. As well, the lattice constant of the first buffer multilayer can increase or decrease layer by layer, enabling the lattice constant of the top layer of the first buffer multilayer to substantially equal that of the bottom of the second buffer multilayer.
Another feature of the present invention is the second buffer multilayer being deposited between the first buffer multilayer and the first cladding layer. The lattice constant of the bottom layer of the second buffer multilayer is substantially equal to the lattice constant of the top layer of the first buffer multilayer. As well, the lattice constant of the second buffer multilayer can increase or decrease layer by layer, enabling the lattice constant of the top layer of the second buffer multilayer to substantially equal that of the first cladding layer.
To achieve the above objects, one aspect of the present invention provides a light-emitting device with reduced lattice mismatch. The light-emitting device comprises a substrate having a first lattice constant, a first buffer multilayer deposited on the substrate, a second buffer multilayer deposited on the first buffer multilayer, and a GaN base epitaxial layer deposited on the second buffer multilayer. The lattice constant of the first buffer multilayer ranges from a first lattice constant at the bottom of the first buffer multilayer to a second lattice constant at the top of the first buffer multilayer. The lattice constant of the second buffer multilayer ranges from a second lattice constant at the bottom of the second buffer multilayer to a third lattice constant at the top of the second buffer multilayer.
According to the present invention, the substrate comprises silicon. Moreover, the first buffer multilayer is represented by general formula BxGa(1xe2x88x92x)P (where 0.02xe2x89xa6xxe2x89xa61). Furthermore, the second buffer multilayer is represented by general formula InyGa(1xe2x88x92y)N (where 0xe2x89xa6yxe2x89xa60.059). Thus, the first lattice constant is about 5.431 xc3x85. The second lattice constant is about 4.538 xc3x85. The third lattice constant is about 4.51 xc3x85.
According to the present invention, the substrate comprises 3Cxe2x80x94SiC. Moreover, the first buffer multilayer is represented by general formula BxGa(1xe2x88x92x)P (where x=1). Furthermore, the second buffer multilayer is represented by general formula InyGa(1xe2x88x92y)N (where 0xe2x89xa6yxe2x89xa60.059). Therefore, the first lattice constant is about 4.32 xc3x85. The second lattice constant is about 4.538 xc3x85. The third lattice constant is about 4.51 xc3x85.
According to the present invention, the substrate comprises GaP. Moreover, the first buffer multilayer is represented by general formula BxGa(1xe2x88x92x)P (where 0xe2x89xa6xxe2x89xa61). Furthermore, the second buffer multilayer is represented by general formula InyGa(1xe2x88x92y)N (where 0xe2x89xa6yxe2x89xa60.059). Therefore, the first lattice constant is about 5.45 xc3x85. The second lattice constant is about 4.538 xc3x85. The third lattice constant is about 4.51 xc3x85.
According to the present invention, the substrate comprises GaAs. Moreover, the first buffer multilayer comprises GaAsxP(1xe2x88x92x) and ByGa(1xe2x88x92y)P (where 0xe2x89xa6Xxe2x89xa61, 0xe2x89xa6yxe2x89xa61). Furthermore, the second buffer multilayer is represented by general formula InzGa(1xe2x88x92z)N (where 0xe2x89xa6zxe2x89xa60.059). Therefore, the first lattice constant is about 5.653 xc3x85. The second lattice constant is about 4.538 xc3x85. The third lattice constant is about 4.51 xc3x85. The GaAsxP(1xe2x88x92x) layer and the ByGa(1xe2x88x92y)P layer are matched, and lattice constant of the top layer of the GaAsxP(1xe2x88x92x) layer and that of the bottom of the ByGa(1xe2x88x92y)P layer are the same, namely a fourth lattice constant.
A detailed description is given in the following embodiments with reference to the accompanying drawings.